1. Field of the Invention
The present invention relates to a micro-mirror device and an associated method, the device adapted so as to change the reflection path of an incident light beam by pivoting a micro-mirror using electrostatic attraction forces. More particularly, the present invention relates to a micro-mirror device and an associated method, the device having an improved structure for restoring the micro-mirror skewed by electrostatic attraction forces to its original position.
2. Description of the Related Art
A general micro-mirror device array is an array in which a plurality of micro-mirrors are installed so as to be pivoted by electrostatic attraction forces, and to reflect incident light beams at different reflection angles depending on pivoting angles or directions. Applications of micro-mirror device arrays include an image displaying apparatus of a projection television and various laser scanning devices such as a scanner, copier, or facsimile machine. In particular, when a micro-mirror device array is employed in an image displaying apparatus, in the micro-mirror device array, micro-mirrors 1 corresponding to the number of required pixels are arranged in an array in a two-dimensional plane, as shown in FIG. 1. The micro-mirrors 1 arranged in an array, so as to correspond to respective pixels as described above are independently pivoted according to an image signal, decide respective reflection angles of incident light beams, and, therefore, can form an image.
Such micro-mirror devices are disclosed in, for example, U.S. Pat. No. 5,331,454 entitled xe2x80x9cLOW RESET VOLTAGE PROCESS FOR DMDxe2x80x9d issued Jul. 19, 1994 and assigned to Texas Instruments Incorporated, and U.S. Pat. No. 5,535,047 entitled xe2x80x9cACTIVE YOKE HIDDEN HINGE DIGITAL MICROMIRROR DEVICExe2x80x9d issued Jul. 9, 1996 and assigned to Texas Instruments Incorporated.
Briefly, as shown in FIG. 2, each of the disclosed micro-mirror devices comprises a substrate 11, first and second address electrodes 13 and 14 provided on the substrate 11, and a micro-mirror disposed to be spaced from and facing the first and second address electrodes 13 and 14.
In the disclosed micro-mirror devices, the micro-mirror 15 is installed on the substrate 11 by means of at least one elastically deformable hinge or post so as to be pivotable, and is maintained in a horizontal position by an elastic restoring force. As the structure of such a hinge or post is described in the above-mentioned inventions, a detailed description thereof is omitted.
In the micro-mirror device having the structure as described above, when respective voltages are applied to the first and second address electrodes 13 and 14 and the micro-mirror 15, the micro-mirror 15 is inclined by electrostatic attraction forces formed according to the differences in electric potentials between the first address electrode 13 and the micro-mirror 15 and between the second address electrode 14 and the micro-mirror 15 to the side having the larger electric potential difference. However, the electrostatic attraction forces must overcome the strength of the hinge or post which tends to keep the micro-mirror in the horizontal position.
That is, as shown in FIG. 3, when voltages V1 and V2 applied to the first and second address electrodes 13 and 14, and voltage V3 applied to the micro-mirror 15 all are zero (0), the micro-mirror 15 is maintained in a horizontal position. Therefore, the distance r1 between the first electrode 13 and the micro-mirror 15 and the distance r2 between the second electrode 14 and the micro-mirror 15 are the same.
On the other hand, when voltages V1, V2, and V3 applied to the first and second address electrodes 13 and 14 and the micro-mirror 15, respectively, have the relationship of V1 less than V2 less than V3, the electrostatic force F1 acting between the first address electrode 13 and the micro-mirror 15 is greater than the electrostatic force F2 acting between the second address electrode 14 and the micro-mirror 15, as shown in FIG. 4. Accordingly, the micro-mirror 15 is pivoted toward the first address electrode 13 side of the substrate 11, and is inclined to a position where the electrostatic force F1 is balanced by the sum of the electrostatic force F2 and a restoring force of the hinge or post, such that the condition of r1 less than r2 is satisfied.
The position of the micro-mirror can also be changed from the position shown in FIG. 4 to the position shown in FIG. 3, or to a position where the micro-mirror is inclined to a direction opposite to the position shown in FIG. 4. These operations of the micro-mirror device are described as follows.
First, when voltages V1, V2, and V3 which all are zero (0) are applied to the first and second address electrodes 13 and 14, and the micro-mirror 15, the position of the micro-mirror 15 changes to the position shown in FIG. 3 under the restoring force of the hinge or post which tends to maintain. the micro-mirror in a horizontal position. In this case, since the dimensions of the hinge or post are on the order of xe2x96xa1m, the strength of the hinge is relatively weak with respect to torque, and the restoring force of the hinge is very weak. Therefore, the time required to change the position of the micro-mirror is longer than the desired time for driving the micro-mirror device, creating a problem in that the micro-mirror device cannot be driven at high speed.
Next, when voltages V1, V2, and V3 which have the relationship of V2 less than V1 less than V3 are applied to the first and second address electrodes 13 and 14 and the micro-mirror 15, respectively, and the micro-mirror 15 is driven to be inclined in the opposite direction, the position of the micro-mirror 15 is changed by the restoring force of the hinge or post and electrostatic forces. In this case, when electrostatic forces F1 and F2 are compared to each other, the fact that the difference between voltages V2 and V3 exceeds the difference between voltages V2 and V3 does not always mean that the electrostatic force F2 is greater than the electrostatic force F1. The reason is that the electrostatic forces F1 and F2 are inversely proportional to respective squares of distances r1 and r2 between the first and second address electrodes 13 and 14 and the micro-mirror 15. Therefore, in this case, until distances r1 and r2 become similar to each other due to the restoring force of the hinge, the effect of reducing the time required to change the position of the micro-mirror 15 by applying voltages having reversed values is insignificant.
Therefore, the micro-mirror device having the structure as described above requires a relatively long time to change the position of a micro-mirror by forming electrostatic attraction forces. Consequently, the driving speed of the micro-mirrors is limited.
To solve the above problem, it is an objective of the present invention to provide a micro-mirror device and an associated method, the device having improved electrode structures, so that the time required to change the position of a micro-mirror, for example, to change from an inclined position of the micro-mirror to an initial position of the micro-mirror, or to an oppositely inclined position of the micro-mirror, can be reduced.
Accordingly, to achieve the above objective, the present invention provides a micro-mirror device including a substrate, address electrodes being provided on the substrate, and a micro-mirror facing the substrate and spaced a predetermined distance from the substrate. The micro-mirror is adapted so that the slope of the micro-mirror can be adjusted by electrostatic attraction forces between the address electrodes and the micro-mirror. The micro-mirror device includes auxiliary electrodes that are formed on and projected from the substrate and the upper portions of which are disposed in the vicinity of the micro-mirror so that restoring force and restoring speed can be enhanced by electrostatic forces of the auxiliary electrodes when an inclined micro-mirror is restored.